The use of semiconductor light modulators is gaining in popularity as a replacement for the laser polygon scanner in xerographic printing processes. A technology of preference, due to its monolithic, semiconductor fabrication process, is the deformable mirror device (DMD). Copending patent application entitled "Spatial Light Modulator Printer and Method of Operation", Ser. No. 07/454,568, filed Dec. 21, 1989 assigned to the common assignee with this patent application, which patent application is hereby incorporated by reference herein, discusses one embodiment of a DMD device using a tungsten light focused via optics on a DMD array. While the invention in that application functions very well, several areas of improvement have become apparent.
These improvements center around the reduction in power consumed, reduction in physical size and uniformity of light modulation across the DMD array. Tungsten sources, like all incandescent filaments, emit light, more or less isotopically, which must be gathered and focused if the light energy is to be efficiently utilized. In addition, a major byproduct of incandescent light is the production of heat which in turn demands an ability of dissipating the heat. This, in turn, requires bulky structures, or plenums, to move the heat away from the source, as well as fans with inherent noise/reliability issues.
Because incandescent light is lambertian in its emission character, the rays must be collected from all sides of the filament and focused, thereby requiring fast optics which again argues for large size and cost, to avoid wasted power due to collection inefficiencies.
Accordingly, one problem in prior art DMD structures is the excessive power required to support the light source coupled with large size, both for the optics and for heat disposition. The aforementioned patent application shows the DMD array positioned in the light energy stream between the light source and the imaging lens aperture. In order to achieve maximum energy transfer, it is necessary to fill the imager aperture with the modulated light image from the DMD array. The imager lens is designed, for cost and other reasons, to be round and thus a square filament image aspect ratio magnified to overfill the imager, must be presented to the lens to insure that the lens is filled. This arrangement, while performing properly, suffers from the problem that not all of the power modulated by the DMD can be passed by the imaging lens, and not all of the light collected by the condenser lenses can be concentrated on the DMD active area.
The latter problem exists for printing systems in that a DMD array is necessarily elongated or linear in aspect, and thus the light pattern reaching the array must be wider than it is high by a significant proportion, compared to either the filament aspect ratio or the aperture of the imager, both of which can be thought of as approximated by a square. Because the source must simultaneously come to focus on the imaging lens circular aperture and at the same time uniformly illuminate the full length of the DMD modulator which is maintained between the aperture of the collimator lens and the focusing lens and since the DMD device is substantially wider than it is high, in order to fully illuminate the array the resulting aspect of the light necessitates considerable width of illumination above and below the active area of the DMD device that is subsequently "wasted".
Another problem is that the focused image from the DMD is continuously projected onto a moving drum. Thus, during the exposure period of a dot-line, for any pixel image location the drum will rotate a given distance and broaden or blur that pixel image. A plot of the light energy on the drum for that pixel location will reveal that the maximum light energy transferred to that pixel will reach a peak level only at the centroid of the pixel and will build up to that point and fall off from that point in the shape of a pyramid. This pyramidal spreading of the pixel energy is not optimum for minimum feature formation and thus reduces the sharpness of the xerographic image in the process direction.
A further problem with existing DMD structures and systems is the fall-off of the light energy on the outer periphery of the DMD array, particularly with elongated arrays, due to the difficulty in collecting and directing light energy from lambertian filament sources.
A still further problem with such existing systems is that optic intensity is not constant or uniform over the whole filament array and there is not a practical method for adjusting the optic characteristics of the light source after manufacture.
Another problem is that the generally available tungsten light sources have filament aspect ratios that are not optimum for DMD light modulator systems. Another problem is the relatively limited lifetime of the tungsten sources.
A final deficiency of the tungsten source is that of the thermal inertia, or lag time of the filament, making it impossible to vary the light output of the source within a printing line time, as is desirable to compensate for specific printing deficiencies.